OK, I see after looking more that it isn't as I suspected initially a test between curlies and parens so much but more on dereferencing...

isequal( [A{ : }] , [A{ : , : }] )

I'm not sure how the parser deals w/ the comma-separated list in an argument, precisely because again it's not documented thoroughly beyond just the description (wonder whether ever heard that from me before? :) ).

The actual content is the same as the [] shows altho clearly the representation seen by the parser passed to the function has something invisible that distinguishes the two. What, precisely, that is, I don't know. Steve?